Fixation of uncemented implants

My thesis postulates that bone ingrowth and direct bone apposition combined with implants engineered to produce interfacial strains lead to beneficial bone remodelling which may result in fixation of joints that will last for the patients life-time. The concept of extra-cortical plate fixation was investigated by assessing the bony response to plates of different design and surface coating. The study found that only one geometric design (holes) significantly increased bone ingrowth into the plate when compared with the control (p=0.01). A crystalline HA coating encouraged significantly greater interfacial contact when compared with a roughened titanium surface (p=0.01), a HA coating of lower crystallinity (p=0.004) and a solution precipitated HA coating (p=0.02). No significant differences were found when bone ingrowth into the plates were compared, except significantly more bone had grown into plates coated with a HA coating of lower crystallinity (p=0.036). Differences in bony reaction induced by the plates of different design were evident and therefore a combination of the correct design and surface coating are required for optimal bone attachment and ingrowth to extra-cortical plates. An experimental goat model was developed to investigate hydroxyapatite coated extra-cortical plate fixation in massive segmental bone tumour replacements. On retrieval, all of the plates were securely fixed by new bone. Bone apposition had occurred through a combination of periosteal bone production, invasion of bone through slots and bone growth over the ends of the plate. It was concluded that due to both mechanical and biological effects, extra-cortical plate fixation generated new bone growth that enhanced fixation and encouraged plate integration into cortical bone. The importance of the implant surface was demonstrated in a series of human autopsy retrieved hip implants. The proximal region of each implant was coated with either a plasma sprayed porous ingrowth surface (plain porous), a HA coated porous surface (porous HA) or a grit blasted surface. Significantly more bone ingrowth (p=0.012) and bone attachment (p<0.05) was measured to the porous HA surface when compared with the plain porous surface. There was no significant difference in bone attachment between the plain porous and grit blasted surfaces. A combination of a HA surface combined with extra-cortical plate fixation has been used to treat a number of bone tumour patients

Finite Element analysis of stress state in the cement of total hip prosthesis with elastomeric stress barrier

In the total hip prosthesis, according to different positions of the patient, there are a variety of loads acting on femoral head which generate stress concentration in the cement called polymethylmethacrylat (PMMA) and consequently in the interfaces stem/cement/bone. This load transfer can provoke loosening of the implant from the femoral bone. This paper focused on optimal stress distribution in the total hip prosthesis and devoted to the development of a redesigned prosthesis type in order to minimize stress concentration in the cement. This study investigated the effect of elastomeric stress barrier incorporated between the stem and femoral head using 3D-finite element analysis. This proposed implant provoked lower load transfer in the cement due to the elastomeric effect as stress absorber.&nbsp; However, the proposed model provided an acceptable solution for load transfer reduction to the cement. This investigation permitted to increase the service life of the total hip prosthesis avoiding the loosening

Failure characteristics of all polyethylene cemented glenoid implants in total shoulder arthroplasty

Total shoulder arthroplasty (TSA) still suffers today from mid-term and long-term complications such as glenoid implant loosening, wear, humeral head subluxation/dislocation and implant fracture. Unlike the hip and knee joint replacements, the artificial shoulder joint has yet to offer a long-term satisfactory solution to shoulder replacement. With loosening being the number one reason for TSA revision, investigating methods of monitoring the glenoid implant loosening and investigate the effects of various design parameters on the loosening behaviour of the glenoid fixation is necessary to explore the problem. Several studies were carried out using in-vitro cyclic testing and FEA to; investigate failure progression and its correlation to quantitative measures in a 2D study (n = 60), investigating key glenoid design features in a 2D (n = 60) and 3D study (n = 20), investigating the validity of using bone substitute foam for studying glenoid fixation in a cadaveric study and investigating any correlation between failure and CT or in-vitro quantitative measures (n = 10). Visible failure was observed, for the first time, correlating to inferior rim displacement and vertical head displacement measures. CT failure was detected in 70% of specimens before visible failure was observed. Out of the design pairs tested; smooth-back/rough-back (range of roughnesses), peg/keel, curved-back/flat-back and conforming/non-conforming, roughening the back-surface to 3.4 μm or more improved fixation performance (p < 0.05). Roughening the back-surface changed the mode of failure from implant/cement failure inferiorly due to tensile/shear stresses, to cement/bone failure superiorly due to compressive/shear loading. Differences in the other design pairs were marked showing peg to perform better than keel, conforming over non-conforming and no difference in curved-back over flat-back, although these differences are marginal. Improvements in the standard testing method have also been suggested

The investigation into modes of failure of total hip replacements

The investigation into modes of failure of total hip replacements was the purpose of this project. This involved the development of a finite element model of a simplified uncemented, simplified cemented and a geometrically real hip in order to study the behaviour of loosening of the prosthesis. Loosening of the femoral and acetabular components in total hip replacements are major long-term complications of hip replacements. Such loosening usually produces substantial loss of supporting bone making revision much more difficult and less successful than primary surgery. Implant loosening can be caused from resorption of the bone around the prosthesis from wear debris, insufficient osseointegration of the bone into the prosthesis and brittle failure of the bone cement

Failure Modes of Biomedical Implants

The metallic biomaterials are very well known among various biomaterials. Stainless steel was used successfully as an implant material in the surgical field, then Vitallium, cobalt based alloys were used as implant materials. Titanium is the newest metallic biomaterial among three main metallic biomaterials, stainless steels, Co-based alloys and titanium alloys. These materials are used for the instrumentation replacing failed hard tissue, for example, artificial hip joints, artificial knee joints, bone plates, dental implants and in similar applications. Biocompatibility characteristics of titanium alloys make it the preferred choice for such applications, additional factors being light weight, excellent mechanical properties, corrosion and wear resistance. The fatigue behavior of biomedical materials is as important as other properties, yet fatigue characteristics are not considered while selecting a particular material for bio applications. This report summarizes the fatigue behavior of biomaterials and also presents a summary from published sources where inferior fatigue resistance lead to the total joint replacement recalls. This paper reviews the fatigue, wear and corrosion resistance of biomaterials that will be useful in the design of bio prostheses

A biomechanical assessment of modular and monoblock revision hip implants using FE analysis and strain gage measurements

<p>Abstract</p> <p>Background</p> <p>The bone loss associated with revision surgery or pathology has been the impetus for developing modular revision total hip prostheses. Few studies have assessed these modular implants quantitatively from a mechanical standpoint.</p> <p>Methods</p> <p>Three-dimensional finite element (FE) models were developed to mimic a hip implant alone (Construct A) and a hip implant-femur configuration (Construct B). Bonded contact was assumed for all interfaces to simulate long-term bony ongrowth and stability. The hip implants modeled were a Modular stem having two interlocking parts (Zimmer Modular Revision Hip System, Zimmer, Warsaw, IN, USA) and a Monoblock stem made from a single piece of material (Stryker Restoration HA Hip System, Stryker, Mahwah, NJ, USA). Axial loads of 700 and 2000 N were applied to Construct A and 2000 N to Construct B models. Stiffness, strain, and stress were computed. Mechanical tests using axial loads were used for Construct A to validate the FE model. Strain gages were placed along the medial and lateral side of the hip implants at 8 locations to measure axial strain distribution.</p> <p>Results</p> <p>There was approximately a 3% average difference between FE and experimental strains for Construct A at all locations for the Modular implant and in the proximal region for the Monoblock implant. FE results for Construct B showed that both implants carried the majority (Modular, 76%; Monoblock, 66%) of the 2000 N load relative to the femur. FE analysis and experiments demonstrated that the Modular implant was 3 to 4.5 times mechanically stiffer than the Monoblock due primarily to geometric differences.</p> <p>Conclusions</p> <p>This study provides mechanical characteristics of revision hip implants at sub-clinical axial loads as an initial predictor of potential failure.</p

Quantification of bone and cement strains surrounding a distal ulnar implant with varying cement-stem interface conditions

Implant loosening following joint replacement surgery is a health-care concern. The role of implant-cement debonding on the propensity of loosening has received limited attention. This thesis examines changes in strains within the cement mantle and bone surrounding distal ulnar implants, as a function of cement-stem interface bonding. A method to embed strain gauges within the cement mantle of the restrictive distal ulnar canal was developed. This technique was applied in 8 cadaveric distal ulnae, where strains were quantified at 2 internal and 5 external (i.e., bone surface) locations under torsion and bending loads with bonded and de-bonded cement-stem interfaces. For a bonded stem, the distal-most external strains increased under all loading scenarios, while proximal internal strains increased only under torsional loading (

Treatment of the fixation surface improves glenoid prosthesis longevity in-vitro

Many commercial cemented glenoid components claim superior fixation designs and increased survivability. However, both research and clinical studies have shown conflicting results and it is unclear whether these design variations do improve loosening rates. Part of the difficulty in investigating fixation failure is the inability to directly observe the fixation interface, a problem addressed in this study by using a novel experimental set-up.Cyclic loading-displacement tests were carried out on 60 custom-made glenoid prostheses implanted into a bone substitute. Design parameters investigated included treatment of the fixation surface of the component resulting in different levels of back-surface roughness, flat-back versus curved-back, keel versus peg and more versus less conforming implants. Visually-observed failure and ASTM-recommended rim-displacements were recorded throughout testing to investigate fixation failure and if rim displacement is an appropriate measure of loosening. Roughening the implant back (Ra > 3 µm) improved resistance to failure (P < 0.005) by an order of magnitude with the rough and smooth groups failing at 8712 ± 5584 cycles (mean ± SD) and 1080 ± 1197 cycles, respectively. All other design parameters had no statistically significant effect on the number of cycles to failure. All implants failed inferiorly and 95 % (57/60) at the implant/cement interface. Rim-displacement correlated with visually observed failure. The most important effect was that of roughening the implant, which strengthened the polyethylene-cement interface. Rim-displacement can be used as an indicator of fixation failure, but the sensitivity was insufficient to capture subtle effects.Level of Evidence: Basic Science Study, Biomechanical Analysis

The Effect of Stem Surface Treatment and Substrate Material on Joint Replacement Stability: An In-Vitro Investigation into the Stem-Cement Interface Mechanics under Various Loading Modes

Mechanical loosening is a common mode of joint replacement failure. For cemented implants, loosening at the implant-cement interface may be affected by stem surface design. Altering the surface topography facilitates the infiltration of bone cement onto the stem, creating a mechanical interlock, improving interface stability. However, few in-vitro studies have investigated this. Therefore, the purpose of this thesis was to investigate the effect of stem surface treatments and substrate materials on stem-cement interface stability in-vitro. Four separate studies were performed to assess the stability of various stem surface treatments, with two substrate materials, under three loading modes. Titanium and cobalt chrome implant stems were custom machined and treated with one of four surfaces: smooth, sintered beads, plasma spray, and circumferential grooves. Sintered bead and plasma sprayed stems were tested in independent torsion, compression and bending; circumferential groove designs were compared in torsion and then compression. All stems were potted in aluminum tubes using PMMA, and loaded cyclically using a materials testing machine. A custom optical tracking system (resolution under 5 μm) was validated for use, and subsequently employed to measure stem-cement interface motion during loading. Overall, results showed surface treatments improved stability, but this was affected by substrate material. Across all loading modes, beaded treatments applied to titanium stems, and plasma spray treatments applied to cobalt chrome stems, improved interface stability and strength when large surface treatment areas were employed. Additionally, the machining of circumferential grooves onto the stem surface improved interface strength in compression, with no influence in torsion. A final study was performed using μ-CT imaging to observe stem and cement motion under bending loads. A custom-built loading device applied static loads to smooth titanium stems, while acquiring CT images of the stem-cement interface. Interface motion was quantified by comparing scans before and after the stem underwent cyclic loading. Results indicated the stem and the surrounding cement had displaced following loading, yet the stems remained relatively stable. These studies offer valuable information regarding the effect of stem surface treatments on stem-cement interface mechanics under various loading modes and will be used in the development of future implant systems